Stream API Fundamentals

The Stream API is a pipeline-based abstraction with three parts: source, lazy intermediate operations, and a terminal operation that triggers execution and produces a result.

A Stream is a pipeline description over a source, not a data structure: it stores nothing, processes elements one at a time via pull-based lazy evaluation, and fuses multiple operations into a single pass.

Streams are lazy because intermediate operations build a pipeline description without executing, and only a terminal operation triggers processing, enabling short-circuit and fusion optimisations.

Streams are single-use: a terminal consumes the stream and any reuse throws IllegalStateException; the workaround is Supplier<Stream<T>> which creates a fresh stream on each .get() call.

Use a traditional loop for index access, in-place mutation, checked exceptions, complex break logic, and nested loops; use streams for declarative pipelines, grouping, and parallelisation.

Streams backed by I/O resources (Files.lines, Files.walk) hold OS file handles that must be closed with try-with-resources; collection streams read from memory and need no closing.

Never mutate shared state inside map or filter; use collect() or reduce() to produce results safely, because parallel streams with shared mutable state cause data corruption.

A Collection stores data (eager, reusable, mutable); a Stream processes data (lazy, single-use, immutable pipeline): Collections hold, Streams flow.